The present invention relates to a folding clasp for a flexible bracelet of which the length of one of the strands can be adjusted by a mechanism for adaptation to the size of the wearer's wrist. In these clasps, the adjustable strand is connected to a first folding blade by the mechanism using one of the strand adjusting orifices, the other, stationary strand being connected by a fastener to the end of the base blade of the clasp either directly or by means of a second folding blade. The blades, also known as folding arms, are connected to one another by rotational shafts in the manner of hinges.
A first type of mechanism comprises a base plate articulated to the end of its folding blade as well as a cap having a cross section in the form of an inverted U extending beyond the base plate. The two edges of one of the ends of the cap are rotatably mounted on the lateral faces of the corresponding end of the base plate, and the free edges of the other end of the cap can be locked by lateral removable means on the lateral faces of the base plate. The adjustable strand passing between the base plate and the cap is held by a stud protruding from the internal face of this base or from the cap when the base or cap is closed on the plate. Such mechanisms are described, for example, in the documents U.S. Pat. No. 1,619,138, CH 632 332, FR 2 699 796 or GB 2 278 148. However, these mechanisms may be too bulky in appearance once closed.
A second type of mechanism comprises a buckle, or loop, having an internal cross section which is slightly greater than that of the strand and which is pivotally mounted on a shaft located at the end of the folding blade. This shaft can be held in cylinders attached to the end of the blade, the shaft or the cylinders constituting the lower cross bar of the buckle, if desired. A central tongue barring the interior of the buckle is also pivotally mounted on this same shaft. Once the adjustable strand has passed into the buckle, the tongue is engaged in one of the adjustment orifices in the strand, then locked in the buckle. This type of mechanism has the advantage of comprising fewer parts, of being more discreet and of resembling a known belt buckle which is simple for the public. However, it is helpful if the tongue can be retracted when the strand is introduced into the buckle and can also be retained in the buckle so as to keep the adjustable strand correctly in the clasp once the clasp has been closed and locked.
The document CH 665 536 discloses such a mechanism in which the tongue can pivot freely forward in the direction of the stationary strand, that is downstream with reference to the direction of insertion of the strand in the buckle, for retraction, but is held toward the rear in the transverse plane of the buckle due to the engagement of its upper end in a recess made in the internal face of the upper cross bar and open downstream. In other words this upper cross bar of the buckle has a lower groove ending with an internal vertical rear wall which prevents the tongue from rocking further upstream.
The document GB 2 293 620 also discloses a mechanism of this type in which the tongue can pivot forwards. On the other hand, the tongue in this mechanism is held in the front transverse plane of the buckle by latching of its upper end in a spherical cap-shaped recess provided in the internal face of the upper cross bar of the buckle.
However, if the initial adjustment of the bracelet is too long, the user may pull on the adjustable strand and advance it relative to the locked clasp which he is also holding. There is therefore a risk that the tongue will rock sufficiently far forwards to leave the orifice in the strand which it is momentarily holding by jamming but which it then releases without the wearer's knowledge owing to vibrations. This risk of the tongue leaving the strand at an inopportune moment is greater if the user merely pulls this strand obliquely in order to open the clasp. There is nothing to prevent the strand from moving back and then leaving the buckle, which may cause the loss of a valuable watch.